1. Field of the Disclosure
The invention relates generally to tube connections, and more particularly, to connecting a flow measurement tube to a base member.
2. Discussion of Related Art
The measurement and control of fluid flow is extremely important in the process industries. Many manufacturing processes require extreme accuracy and repeatability in fluid delivery, and thus demand that the mass flow rate of process fluids be precisely measured and controlled. Various technologies are known for measuring mass flow. For example, mass flow measurement based on the Coriolis force effect provides a direct measurement of mass flow. In the case of the typical Coriolis force flow sensor, a flow sensing tube, through which fluid flow is to be established, is vibrated. Often the tube is in the shape of one or more loops. The loop shape is such that the mass flow vector is directed in opposite directions at different parts of the loop. The tube loops may, for example, be “U” shaped, rectangular, triangular or “delta” shaped or coiled. In the special case of a straight tube, there are two simultaneous angular velocity vectors that are coincident to the anchor points of the tube while the mass flow vector is in a single direction.
The angular velocity vector changes directions since, in a vibrating system, the direction of rotation changes. The result is that, at any given time, the Coriolis force is acting in opposite directions where the mass flow vectors or the angular velocity vectors are directed in opposite directions. Since the angular velocity vector is constantly changing due to the vibrating system, the Coriolis force is also constantly changing. The result is a dynamic twisting motion being imposed on top of the oscillating motion of the tube. The magnitude of twist is proportional to the mass flow for a given angular velocity.
A thermal mass flow instrument measures flow by routing a small portion of the fluid stream through a flow sensing tube. Heat is applied at the midpoint of the sensing tube, with temperature sensors located on either side of the heater. Each temperature sensor measures the temperature of the fluid at its respective location. The first temperature sensor measures the temperature upstream of the heater. The second temperature sensor measures the temperature downstream of the heater and reflects a temperature corresponding to the fluid as heated by the heater. The temperature difference of the fluid on either side of the heater is proportional to the mass flow rate.
The flow sensing tube in such flow measurement devices is typically connected to a base member, typically at or near the inlet and outlet ends of the tube. To provide reliable operation, the tube connections must be solid and leak-free. Typically, the flow tube is brazed to the base member. Brazing produces a solid and leak free connection but may have inferior corrosion resistance when compared to the tube material. Welding is a preferred joining technique. However, known manufacturing processes and tolerances often make it difficult to achieve satisfactory welded tube connection joints, especially in low-flow applications, which require very small flow sensing tubes.
The present invention addresses shortcomings associated with the prior art.